(Note: For anyone who is wondering about the 1967 Fantastic Four series the Triton we see in that show is actually a blue-skinned expy of Namor the Sub-Mariner. However since Sub Mariner has his show in Marvel Super Heroes at the time Hanna Barbera couldn't directly use Namor so they colored their take on the character blue and coincidentally name him Triton.)
Triton is one of the members of the Inhumans whom became allies of the Fantastic Four and his Comic Book Debut was in Fantastic Four #45 in December of 1965. Triton is the Brother of Karnak and a member of the Inhumans Royal Family in which his powers are to be able to breath underwater and is strong enough to stand the cold temperatures and high pressure of the ocean deep. Unlike a lot of aquatic characters in comic books Triton's fish-like appearance was due to his body being mutated by the Terrigen Mist though whether he needs a device to breath out of water or not depends on the story. Triton is a member of the Inhuman Royal Family and has aided the Fantastic Four but he is also a good friend of Namor the Sub Mariner and has been a member of Namor's Deep Six in the past.